1. Field of the Invention
This invention relates to a white balance correction circuit and more particularly to a white balance correction circuit adapted for image sensing apparatuses such as a color video camera, an electronic still camera, etc.
2. Description of the Related Art
The human eye has a color constancy which causes a red object to appear to be red and a white object to be white even in the event of a change occurred in the color of a light source. This is considered to be attributable to cancellation of the color of ambient light by the visual sensation information processing system of the brain. Meanwhile, the conventional color video camera has been arranged to measure the color of the light source and to remove the adverse effect of the measured color by varied methods (white balance correcting methods).
FIG. 1 of the accompanying drawings is a block diagram showing the arrangement of a color video camera which incorporates therein the conventional white balance correction circuit. An image of the object to be photographed is formed on an image sensor 12 through a photo-taking lens 10. The image sensor 12 then produces signals R, G and B. The output signals R and B of the image sensor 12 are supplied to a signal processing circuit 18 respectively via variable gain amplifiers 14 and 16 which are provided for white balance correction. The output signal G of the image sensor 12 is supplied directly to the signal processing circuit 18. The signal processing circuit 18 is arranged to produce a video signal by performing a known video signal processing operation on these inputs including clamping, gamma correcting and synchronizing signal adding processes.
The gains of the amplifiers 14 and 16 are controlled in the following manner: The ambient light around the camera is diffused by a diffusing plate 20 before it comes to a colorimetric sensor 22. The colorimetric sensor 22 is arranged to produce color signals Rm, Gm and Bm which represent the amounts of three primary color components, respectively. The outputs Rm, Gm and Bm of the colorimetric sensor 22 are logarithmically compressed by logarithmic compression circuits 24R, 24G and 24B, respectively. A subtracter 26 is arranged to subtract the output logGm of the logarithmic compression circuit 24G from the output logRm of the logarithmic compression circuit 24R. A subtracter 28 is arranged to subtract the output logGm of the logarithmic compression circuit 24G from the output logBm of the logarithmic compression circuit 24B. The gain of the amplifier 14 is controlled by the output log(Rm/Gm) of the subtracter 26. The gain of the amplifier 16 is controlled by the output log(Bm/Gm) of the subtracter 28. The white balance correction is thus controlled.
FIG. 2 shows another example of the conventional white balance correction arrangement. In this case, the outputs R, G and B of an image sensor 12 are integrated by integrating circuits 30R, 30G and 30B for one picture and are then logarithmically compressed by logarithmic compression circuits 32R, 32G and 32B. Like the above-stated subtracter 26, a subtracter 34 is arranged to subtract the output of the logarithmic compression circuit 32G from the output of the logarithmic compression circuit 32R. Like the subtracter 28, a subtracter 36 subtracts the output of the logarithmic compression circuit 32G from the output of the logarithmic compression circuit 32B. The gain of an amplifier 14 is controlled by the output of the subtracter 34. The gain of another amplifier 16 is controlled by the output of the subtracter 36.
The conventional color video camera which is arranged as shown in FIG. 1 is, however, incapable of making accurate white balance correction in cases where the object to be photographed and the diffusing plate 20 are illuminated by different light sources, where a picture of an outdoor scene is taken from the inside of a room or where the diffusing plate 20 is inadvertently blocked from light by a finger or the like.
In the case of FIG. 2, the conventional camera is based on the concept that an integrating action on the color of an object brings it close to a neutral gray color. However, there are some objects to which this concept is not applicable. For example, in a case where an object of a specific color is occupying a larger portion of the picture than other objects like in taking a picture of a person located in front of a red wall, accurate white balance correction is impossible by the camera of FIG. 2.
The conventional color video cameras described above thus have their advantages and disadvantages according to the conditions under which they are to be used. The conditions under which they are advantageously usable are limited.
FIG. 3 is a block diagram showing a further example of a color video camera which incorporates therein the conventional white balance correction circuit. The illustration includes an image sensor 110; a luminance and chromaticity forming circuit 112; white balance correction amplifiers 114 and 116; and an encoder 118. The encoder 118 is arranged to form a standard TV signal (of the NTSC system, for example) from a luminance signal output from the luminance and chromaticity forming circuit 112 and color difference signals which have been white-balance-corrected by the white balance correction amplifiers 114 and 116. A white balance correction signal forming circuit 122 is arranged to form white balance correction signals from the output of a color temperature sensor 120.
The operation of the color video camera of FIG. 3 is briefly described as follows: Light incident on the image sensor 110 is photo-electric converted. An electrical signal thus obtained is supplied to the luminance and chromaticity forming circuit 112. The luminance and chromaticity forming circuit 112 then outputs a luminance signal Y.sub.H to a signal line 112A; a low-band luminance signal Y.sub.L to a signal line 112B; a color-difference signal (R-Y.sub.L) to a signal line 112C; and a color-difference signal (B-Y.sub.L) to a signal line 112D. The color-difference signals of the signal lines 112C and 112D are respectively supplied to the white balance correction amplifiers 114 and 116.
The white balance correction signal forming circuit 122 forms white balance correction signals on the basis of the output of the color temperature sensor 120. In accordance with the white balance correction signals, the amplifiers 114 and 116 add or subtract the low-band luminance signal Y.sub.L of the signal line 112B to or from the color-difference signals supplied to them. As a result, the amplifiers 114 and 116 output white-balance-corrected color-difference signals, respectively.
The encoder 118 forms a standard TV signal from the luminance signal Y.sub.H output from the luminance and chromaticity forming circuit 112 and the white-balance-corrected color-difference signals (R-Y.sub.L) and (B-Y.sub.L) output from the amplifiers 114 and 116. The encoder 118 thus outputs the standard TV signal. However, since the example of the conventional color video camera of FIG. 3 has its white balance correction system arranged in an open loop, it is inferior in the absolute degree of accuracy.
To eliminate this shortcoming, another example of the conventional color video camera has a white balance correction system arranged in a closed loop as shown in FIG. 4. In FIG. 4, the same component parts as those of FIG. 3 are indicated by the same reference numerals as in FIG. 3. The example includes a white balance correction signal forming circuit 124 which corresponds to the white balance correction signal forming circuit 122 of FIG. 3. The conventional camera example differs from the camera of FIG. 3 in that the white balance correction signals are formed with reference to the outputs of the amplifiers 114 and 116.
The color video camera of FIG. 4 operates as follows: The luminance and chromaticity forming circuit 112 is arranged like the camera of FIG. 3 to output the luminance signals Y.sub.H and Y.sub.L and the color-difference signals (R-Y.sub.L) and (B-Y.sub.L). The color-difference signals (R-Y.sub.L) and (B-Y.sub.L) are supplied respectively to the amplifiers 114 and 116. Like in the case of FIG. 3, the amplifiers 114 and 116 are arranged to respectively add or subtract the low-band luminance signal Y.sub.L to or from the color-difference signals (R-Y.sub.L) and (B-Y.sub.L) in accordance with white balance correction signals output from the white balance correction signal forming circuit 124. The amplifiers 114 and 116 thus output white-balance-corrected color-difference signals.
The white balance correction signal forming circuit 124 compares a mean value of each of the white-balance-corrected color-difference signals obtained by the amplifiers 114 and 116 with an achromatic color level of each of the color-difference signals. The circuit 124 then forms the white balance correction signals and supplies them to the amplifiers 114 and 116 in such a way as to make the levels of the color-difference signals equal to each other. The white balance correcting operation is thus performed in a closed loop.
The conventional color video camera of FIG. 4 is thus arranged to have the white balance correcting system operate with a high degree of accuracy on account of its closed loop arrangement. However, in a case where the color temperature (or spectral distribution) of an illumination light on the object is not reflected by the light incident upon the image sensor 110 like in the event of a monochromatic colored object, the camera makes a faulty white balance correction.